Only create changed buffer if it already exists (#13242)

# Objective

- `DynamicUniformBuffer` tries to create a buffer as soon as the changed
flag is set to true. This doesn't work correctly when the buffer wasn't
already created. This currently creates a crash because it's trying to
create a buffer of size 0 if the flag is set but there's no buffer yet.

## Solution

- Don't create a changed buffer until there's data that needs to be
written to a buffer.

## Testing

- run `cargo run --example scene_viewer` and see that it doesn't crash
anymore

Fixes #13235

crates/bevy_render/src/render_resource/buffer_vec.rs

@@ -43,7 +43,7 @@ pub struct RawBufferVec<T: NoUninit> {
     item_size: usize,
     buffer_usage: BufferUsages,
     label: Option<String>,
-    label_changed: bool,
+    changed: bool,
 }
 
 impl<T: NoUninit> RawBufferVec<T> {
@@ -55,7 +55,7 @@ impl<T: NoUninit> RawBufferVec<T> {
             item_size: std::mem::size_of::<T>(),
             buffer_usage,
             label: None,
-            label_changed: false,
+            changed: false,
         }
     }
 
@@ -93,7 +93,7 @@ impl<T: NoUninit> RawBufferVec<T> {
         let label = label.map(str::to_string);
 
         if label != self.label {
-            self.label_changed = true;
+            self.changed = true;
         }
 
         self.label = label;
@@ -115,16 +115,16 @@ impl<T: NoUninit> RawBufferVec<T> {
     /// the `RawBufferVec` was created, the buffer on the [`RenderDevice`]
     /// is marked as [`BufferUsages::COPY_DST`](BufferUsages).
     pub fn reserve(&mut self, capacity: usize, device: &RenderDevice) {
-        if capacity > self.capacity || self.label_changed {
-            self.capacity = capacity;
         let size = self.item_size * capacity;
+        if capacity > self.capacity || (self.changed && size > 0) {
+            self.capacity = capacity;
             self.buffer = Some(device.create_buffer(&wgpu::BufferDescriptor {
                 label: self.label.as_deref(),
                 size: size as BufferAddress,
                 usage: BufferUsages::COPY_DST | self.buffer_usage,
                 mapped_at_creation: false,
             }));
-            self.label_changed = false;
+            self.changed = false;
         }
     }
 

crates/bevy_render/src/render_resource/storage_buffer.rs

@@ -250,7 +250,7 @@ impl<T: ShaderType + WriteInto> DynamicStorageBuffer<T> {
         let capacity = self.buffer.as_deref().map(wgpu::Buffer::size).unwrap_or(0);
         let size = self.scratch.as_ref().len() as u64;
 
-        if capacity < size || self.changed {
+        if capacity < size || (self.changed && size > 0) {
             self.buffer = Some(device.create_buffer_with_data(&BufferInitDescriptor {
                 label: self.label.as_deref(),
                 usage: self.buffer_usage,

crates/bevy_render/src/render_resource/uniform_buffer.rs

@@ -295,7 +295,7 @@ impl<T: ShaderType + WriteInto> DynamicUniformBuffer<T> {
             .checked_mul(max_count as u64)
             .unwrap();
 
-        if capacity < size || self.changed {
+        if capacity < size || (self.changed && size > 0) {
             let buffer = device.create_buffer(&BufferDescriptor {
                 label: self.label.as_deref(),
                 usage: self.buffer_usage,
@@ -336,7 +336,7 @@ impl<T: ShaderType + WriteInto> DynamicUniformBuffer<T> {
         let capacity = self.buffer.as_deref().map(wgpu::Buffer::size).unwrap_or(0);
         let size = self.scratch.as_ref().len() as u64;
 
-        if capacity < size || self.changed {
+        if capacity < size || (self.changed && size > 0) {
             self.buffer = Some(device.create_buffer_with_data(&BufferInitDescriptor {
                 label: self.label.as_deref(),
                 usage: self.buffer_usage,

crates/bevy_render/src/view/mod.rs

@@ -429,6 +429,7 @@ pub struct ViewUniforms {
 impl FromWorld for ViewUniforms {
     fn from_world(world: &mut World) -> Self {
         let mut uniforms = DynamicUniformBuffer::default();
+        uniforms.set_label(Some("view_uniforms_buffer"));
 
         let render_device = world.resource::<RenderDevice>();
         if render_device.limits().max_storage_buffers_per_shader_stage > 0 {